You have a choice between Gaia, Hastur, and Titania. If your goal is to simply use the team against Thunder, any of these will do. Titania is good for Burst and Berserk builds, but a Berserk build requires SSR Cybele to work effectively. Hastur is an Oberon replacement and can fit in multiple builds, but won't provide much value over Oberon when you're lacking debuffs. Gaia is excellent against Thunder, but tends to be replaced by Titania in offensive builds. Against elements other than Thunder, you'd probably be stuck with Mordred for the time being regardless of pick.